This invention relates to methods for preparing titanium dioxide pigments at lower reaction temperatures than those of prior art methods. The titanium dioxide pigments of the present invention possess unexpected improvements in gloss when prepared at low temperatures.
Titanium dioxide (TiO2) pigments are important in the manufacture of paints, plastics, and coatings. There has been a considerable research effort to make titanium dioxide pigments to improve the gloss of paints, plastics, or coatings.
In general, pigments useful in the manufacture of plastics, paints and coating are comprised of a base titanium dioxide pigment having layers or coatings of various inorganic compounds. Such compounds include alumina, silica, zirconia, phosphates, and the like. Typically, coating titanium dioxide pigments with silica and alumina involves first preparing an aqueous slurry of titania particles, at a temperature of above 75xc2x0 C. and subsequently adding a soluble silica compound to the slurry. The pH of the slurry is then adjusted to between about 8 and 10 to initiate the deposition of a dense silica layer. After silica deposition, an alumina compound is added to the slurry and deposited on the titania particles.
Many prior art references disclose that to prepare commercially acceptable coated titanium dioxide pigment, careful control of both the temperature above 75xc2x0 C. and pH is required. Conventional prior art wisdom teaches that coating titanium dioxide at temperatures below 75xc2x0 C. will produce a commercially unacceptable pigment with poor gloss and/or durability.
Many commercial treatment systems, i.e. single or multistage systems are designed to have maximum heating temperatures of 75xc2x0 C. or less. Accordingly, prior art titanium dioxide pigments coated at temperatures above 75xc2x0 C. can not be produced in such systems.
It would be beneficial to produce coated titanium dioxide pigments at lower reaction temperatures (e.g. below 75xc2x0 C.) which utilize less overall heat energy while improving gloss and maintaining durability. Such pigments can be prepared in reactors that are designed to run at temperatures lower than 75xc2x0 C. as well as reactors that run at higher temperatures.
Based on the foregoing, there is a need in the art for methods of preparing titanium dioxide pigments with commercially acceptable gloss and durability at temperatures below about 75xc2x0 C. It has been unexpectedly discovered that coated titanium dioxide pigments produced at temperatures below about 75xc2x0 C. will have commercially acceptable or improved gloss, while maintaining durability.
The present invention provides a titanium dioxide pigment with improved gloss produced using less overall heat energy. Accordingly, the present invention provides a method of preparing a titanium dioxide pigment having improved gloss properties comprising the steps of: a) heating an aqueous slurry of base titanium dioxide pigment to a temperature below about 75xc2x0 C.; b) coating the base titanium dioxide pigment with a silica compound at a temperature below about 75xc2x0 C. to form a silica coated titanium dioxide pigment; and c) coating the silica coated titanium dioxide pigment with an alumina compound at a temperature below about 75xc2x0 C. to form the titanium dioxide pigment having improved gloss and acceptable durability.
In an alternate embodiment, the present invention provides a method of enhancing gloss of a titanium dioxide pigment comprising the step of: treating the titanium dioxide pigment with a silica compound and an alumina compound at a temperature below about 75xc2x0 C.
In yet another embodiment, the present invention provides a method of treating titanium dioxide by coating the titanium dioxide with a silica compound and an alumina compound to form treated titanium dioxide having enhanced gloss, the improvement comprising the step of: maintaining the temperature of the titanium dioxide during the method at a temperature below about 75xc2x0 C.
In a preferred embodiment, the present invention provides a method for preparing titanium dioxide pigment having improved gloss at reduced temperatures, comprising the steps of: a) heating a slurry of titanium dioxide pigment under agitation to a temperature between about 60xc2x0 C. to below about 75xc2x0 C.; b) adding a silica compound to the slurry; c) curing the silica to form a silica coated titanium dioxide pigment; d) adjusting the pH of the slurry to a value of from about 6 to about 7; e) adding an alumina compound to the silica-treated slurry while maintaining the pH at a value of about 6.5; f) curing the alumina compound under agitation to form an alumina coating; g) adjusting the pH of the slurry to a value of from about 5.5 to about 6.5; h) discharging, washing and filtering the pigment; and i) drying, micronizing and treating the pigment with trimethylolpropane.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.